1. Field of the Invention
The invention relates to light emitting devices and more particularly to light emitting devices including at least one light-emitting diode and phosphor, the phosphor including lead and/or copper doped chemical compounds and converting the wavelength of light.
2. Description of the Related Art
Light emitting devices (LEDs), which used to be used for electronic devices, are now used for automobiles and illumination products. Since light emitting devices have superior electrical and mechanical characteristics, demands for light emitting devices have been increased. In connection to this, interests in white LEDs are increasing as an alternative to fluorescent lamps and incandescent lamps.
In LED technology, solution for realization of white light is proposed variously. Normally, realization of white LED technology is to put the phosphor on the light-emitting diode, and mix the primary emission from the light emitting diode and the secondary emission from the phosphor, which converts the wavelength. For example, as shown in WO 98/05078 and WO 98/12757, use a blue light emitting diode, which is capable of emitting a peak wavelength at 450-490 nm, and YAG group material, which absorbs light from the blue light emitting diode and emits yellowish light (mostly), which may have different wavelength from that of the absorbed light.
However, in such a usual white LED, color temperature range is narrow which is between about 6,000-8,000K, and CRI (Color Rendering Index) is about 60 to 75. Therefore, it is hard to produce the white LED with color coordination and color temperature that are similar to those of the visible light. It is one of the reasons why only white light color with a cold feeling could be realized. Moreover, phosphors which are used for white LEDs are usually unstable in the water, vapor or polar solvent, and this unstableness may cause changes in the emitting characteristics of white LED.